In the towing of farm equipment by a tractor it is often desirable to move the towed equipment as rapidly as possible. This is particularly true in the case of the towing or pulling of a plow. However, a plow or other towed apparatus presents a certain resistance to movement, while such towing is often accomplished under conditions of poor traction in which the tractor drive wheels tend to spin or slip if an effort is made to apply too much power. Thus, applying too much power, or attempting to attain maximum speed of travel may end up in inefficient operation, wasting gross amounts of fuel, and hence increasing costs, or even increasing wheel slippage to a point where travel is slower than if wheel slippage is substantially avoided.